1. Field of the Invention
The present invention relates to a method of controlling a system for correcting skew of a web of material being fed through a machine. The present invention also relates to a system for correcting skew of a web of material being fed through a machine. The present invention further relates to an apparatus, in particular for printing on a web of material, comprising a sheet feed path including a registration module. The invention also relates to a computer program.
2. Description of Background Art
Respective examples of such a method, system and apparatus are known. For example, U.S. Pat. No. 5,917,727 discloses a registration system that positions a sheet being transported along a path so that the sheet is properly aligned with a printer. In operation, first and second stepper motors rotate at a substantially similar and predetermined speed so that first and second roller pairs rotate and transport the sheets. As the sheet is being transported, it will pass through the first and second roller pairs and the leading edge will trip first and second sensors. A control system can measure the interval between the moments when the first and second sensors are tripped. In response, the control system will create a speed differential between the first and second stepper motors by increasing the speed of one stepper motor and decreasing the speed of the other stepper motor. The controller will also cause a phase differential between steps in the first and second stepper motors. The magnitude of the speed change for the first and second stepper motors is approximately the same, so that the mean speed of the sheet will remain substantially the same as it is being rotated. Once the sheet is shifted to a second position wherein the leading edge is substantially perpendicular to the transport path, the first and second stepper motors are returned to substantially the same speed and the phase differential between the steps is returned to approximately zero.
A problem of the known system is that it only allows a skew correction by turning one of the first and second roller pairs over a larger distance than the other with the difference being commensurate with an integer number of steps of a stepper motor. Thus, the displacement provided in response to one step pulse by a stepper motor and the transmission system connecting it to the roller it is driving limits the resolution. An increase in resolution is attainable by using an appropriate transmission ratio in the transmission system. However, this only functions when the stepper motor is driven at high frequencies. Otherwise, skew correction would take too long. The cost of using high-frequency drivers, taking into account that the first and second stepper motors are driven independently, is an obstacle to adaptation of the transmission system in this way.